1. Field
The present disclosure relates to tires for vehicles of the heavy goods vehicle type and more particularly to the treads of such tires intended to be fitted to the driven axles of heavy goods vehicles.
2. Description of Related Art
In general, a tire for a heavy goods vehicle such a lorry, bus, tractor, comprises a carcass reinforcement comprising a plurality of reinforcers arranged radially, namely making an angle of or close to 90° with the circumferential direction, this carcass reinforcement being itself surmounted by a crown reinforcement extending in the circumferential direction. This crown reinforcement is itself surmounted on its radially outer surface by a tread made with at least one rubber compound of which the radially outermost part forms a tread surface intended to come into contact with the roadway when the tire is being driven on.
In combination with this internal structure of the tire it is known practice to provide the tread, namely that part of the tire that is intended to come into contact with the ground during driving and to be worn away during driving, with a tread pattern made up of raised elements delimited by grooves whether these be orientated circumferentially, transversely or obliquely. The purpose of such a tread pattern is to give the tread good performance when driving on a dry roadway and on a roadway covered with water, notably in the rain.
In order to improve tread performance without, however, excessively lowering the shear rigidity of said treads, it is known practice to form on the tread surface a plurality of edge corners oriented transversely or obliquely in order to cut into the film of water on a roadway to ensure good contact between the tread and the roadway. One way of obtaining such edge corners is to provide the tread with a plurality of cuts, these cuts taking the form of grooves or the form of sipes. In the present application, sipes are distinguishable from grooves in that sipes have a width suited to allowing, during running, at least partial contact between the opposing walls that delimit these sipes and notably when these sipes are in the contact patch in which the tire makes contact with the ground, which is something that the grooves do not do under normal conditions of use of the tire.
Combined with this need to improve grip performance through the presence of edge corners formed by the transverse cuts, it is also a requirement that tread performance be lasting, namely for satisfactory performance to be achieved even following more or less pronounced partial wearing. The partial wearing of a tread means a degree of wear that corresponds to a tread thickness at most equal to the total thickness of tread that can be worn away before the tire has to be changed in order notably to comply with regulations.
Forming a plurality of cuts in a tread, while advantageous in improving grip performance notably on wet ground, results in a significant drop in the rigidity of the tread, through the absence of material, and this may prove disadvantageous in terms of wear. This is more especially the case when the tread has to offer the user lasting performance and the total thickness to be worn away has accordingly to be relatively great. On average, for a tire intended to be mounted on a driven axle, this thickness is at least equal to 18 mm, this thickness taking account of the additional thickness needed for what is referred to as the “regrooving” operation.
Patent application WO 02/38399-A2 describes a tread for a heavy goods vehicle tire, this tread comprising a plurality of circumferential and transverse grooves. The transverse grooves are formed of an alternation of void zones and incisions so that it has a volume of voids that open onto the tread surface in the new state and a volume of hidden voids, these hidden voids being intended to open after that same tread has become partially worn. The presence of hidden voids—which appear with wear, means that the rigidity in the initial state can be greater but with grip performance still being assured regardless of the degree of tread wear.
Patent application WO 2011/039194-A1 describes a new type of tread block whereby lasting tread performance is obtained through special management of the void volumes in the said tread.
In that patent application WO 2011/039194-A1, there is described a tread for a heavy goods vehicle tire which tread is provided with at least one groove delimited by opposing lateral walls, this groove opening onto the tread surface discontinuously as a plurality of portions which are open to the outside of the tread, the lateral walls being connected by a part that forms the bottom of the groove. These open portions comprise two ends continuing under the tread surface in the form of first channels, the channels having inlets connected to the ends of the open portions. Each of these grooves offers a drainage capacity in the new state because of the continuity with the channels; this drainage capacity is fulfilled in the worn state when, in combination with these first channels, continuous second channels are wholly formed under the tread surface, these second channels being connected or not connected to the first channels. In this way it is possible to limit the reduction in the tread rigidity, notably shear rigidity under tangential loading that occurs during running in contact with a roadway while at the same time maintaining a suitable capacity of the grooves to remove the water present on the roadway.
By comparison with the prior art, this type of groove—while providing the minimum level of drainage thanks to the presence of a suitable voids volume open to the tread surface—allows the tread to maintain rigidities, notably shear rigidity, which are greater than that of treads provided with grooves of depths at least equal to the thickness of material to be worn away during running.